Vibrations coupled, carbonyl

During this study, we found that the exciton model underestimates the splitting between the coupled states by about a factor of four, indicating that interactions other than dipole-dipole coupling act between adjacent carbonyl groups. These interactions are most likely straightforward vibrational coupling terms, as discussed at the end of Section 3. 

The first observation of VCD in small model DNA molecules, and in large deoxynucleic acid models, was reported by us in 1989 [18]. The VCD signals in the 1550 to 1750 cm"1 spectral region were found to be similar to those observed for the RNA reported earlier. In addition, our first report on DNA models also contained the observation of B and Z form DNA, along with the DECO equations and model calculations of the helical polymers. Subsequently, VCD was reported for poly(dG-dC) poly(dG-dC), poly(dG) poly(dC), poly(dA-dT) poly(dA-dT) and poly(dA) poly(dT) in the B-conformation in buffered, aqueous solution [22]. Differences were observed for DNA models, depending on the base composition and sequence, and the observed results were quantitatively interpreted in terms of the exciton model for coupled carbonyl stretching vibrational states. 

The peaks between 800 and 600 cm are caused by vibrations of the bases which are coupled to sugar vibrations and out-of-plane carbonyl bending vibrations. A mode attributed to a guanine imidazole ring vibration coupled via the Cl -N9 linkage to a sugar vibration in particular is extremely sensitive to the conformation. It therefore gives evidence of transitions between the different helical families of nucleic acids (Secs. 4.7.1.3.1, 4.7.1.3.2). 

One can calculate overtone, combination, and difference bands by directly manipulating frequencies in wavenumbers via multiplication, addition, and subtraction, respectively. When a fundamental vibration couples with an overtone or combination band, the coupled vibration is called Fermi resonance. Again, only certain combinations are allowed. Fermi resonance is often observed in carbonyl compounds. 

In some aromatic acid chlorides one may observe another rather strong band, often on the lower-frequency side of the C=0 band, which makes the C=0 appear as a doublet. This band, which appears in the spectrum of benzoyl chloride (Fig. 2.56) at about 1730 cm is probably a Fermi resonance band originating from an interaction of the C=0 vibration, with an overtone of a strong band for 1-C stretch often appearing in the range from 900 to 800 cm . When a fundamental vibration couples with an overtone or combination band, the coupled vibration is called Fermi resonance. The Fermi resonance band may also appear on the higher-frequency side of the C=0 in many aromatic acid chlorides. This type of interaction can lead to splitting in other carbonyl compounds, as weU. 

When several carbonyl ligands of the same type are present, vibrational coupling [23] will produce a more complicated IR spectrum in the 1900-2100 cm region. This is illustrated in Fig. 7.6. 

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